1. Technical Field
The present invention relates to a method of preparing a sensitizer particle dispersion for a thermosensitive recording medium composed mainly of stearic acid amide, a sensitizer particle dispersion prepared thereby, a mixed dispersion composition for a thermosensitive recording layer using the sensitizer particle dispersion, and a thermosensitive recording medium using the mixed dispersion composition.
2. Description of the Related Art
A thermosensitive recording medium, which makes use of the color-developing reaction of a dye, a color developer and a sensitizer induced by heat, is inexpensive, and is thus useful in facsimile systems, printers, etc. and widely utilized in labels, tickets and so on.
A typical thermosensitive recording medium is manufactured by applying a composition containing an electron donating dye, an electron accepting color developer, etc. on a support such as paper, a film, etc. and then drying it. When the surface of the layer thus applied (which is a thermosensitive recording layer) is subjected to Joule heating from a thermal head, the dye and the color developer, which are distributed in the applied layer, are melt-reacted, thus forming a color developed phase.
However, a commercially available thermosensitive recording medium may include, in addition to the dye and the color developer, a sensitizer (a recording sensitivity improver) for increasing heat response to achieve high-speed recording and low energy consumption (energy saving).
Examples of the sensitizer may include stearic acid amide, 1,2-bis(phenoxy)ethane, 1,2-bis(3-methylphenoxy)ethane, 1,2-bis(4-methylphenoxy)ethane, p-benzylbiphenyl, di-p-methylbenzyl oxalate, β-naphthylbenzylether, diphenylsulfone, waxes, etc.
When the sensitizer is contained in the thermosensitive recording layer, it is distributed in the form of particles. As the size of the particles thereof is reduced, the sensitizer may exhibit superior heat melting properties and may thus effectively function.
As such, the sensitizer having a small particle diameter may be contained in the thermosensitive recording layer in such a manner that a sensitizer, which was previously made in the form of particles, is dispersed in a thermosensitive recording layer, or that a sensitizer is finely milled to an average particle diameter of 0.40 μm, 0.25 μm or 0.10 μm together with a dye or a color developer using a sand grinder (a wet-mill), etc. and is then contained in the thermosensitive recording layer (Patent Document 1).
However, at present the average particle diameter of the sensitizer contained in the thermosensitive recording layer and provided practically falls in the range of about 1˜3 μm. This is because particles having a particle diameter of 1 μm or less have to be made using a specific particle forming device and the formation thereof requires high energy and long treatment time, undesirably resulting in high treatment costs.
Particularly useful as the sensitizer, stearic acid amide is employed in general-grade thermosensitive recording media because it is cheap. It is also known to manifest excellent functionality as a sensitizer for a thermosensitive recording medium containing 4,4′-dihydroxydiphenylsulfone as a color developer.
In order for the thermosensitive recording medium to develop high-sensitivity color even in the low energy range, the dye and the color developer in particle form distributed in the thermosensitive recording layer have to be instantly melt-reacted. To this end, however, stearic acid amide having a melting point of 102° C., which acts as a solvent for a dye and a color developer, should have a small particle diameter (e.g. 1 μm or less) to increase heat melting properties.
As for the reduction in the particle diameter of stearic acid amide to form stearic acid amide particles, the use of a sand grinder (a wet-mill) is disclosed. However, this method is inefficient and is not profitable because a long period of time is required to form stearic acid amide particles and also because the resulting dispersion has an unsatisfactory concentration of about 10% (Patent Document 2).
Furthermore, as for the reduction in the particle diameter of stearic acid amide, emulsifying stearic acid amide to form an emulsion thereof is disclosed (Patent Document 3).
According to this method, water, stearic acid amide and an emulsifier are placed in a pressure vessel, heated to 100° C. or more (e.g. 120° C.) and then processed at a pressure of 20 MPa using a high-pressure homogenizer, and thereby stearic acid amide may be provided in the form of an emulsion containing fine and uniform particles having an average particle diameter of 0.7 μm or less with good stability.